feat: migrate from SQLite to PostgreSQL - complete schema and API updates

- Remove better-sqlite3, add pg driver
- Convert schema to PostgreSQL types (serial, timestamp, boolean, jsonb)
- Generate fresh PostgreSQL migrations
- Update database connection layer with pg.Pool
- Fix all API routes: remove JSON.parse/stringify, use native timestamps and booleans
- Update drizzle.config.ts and .env.example for PostgreSQL

src/app/api/export/route.ts

@@ -45,7 +45,7 @@ export async function GET(request: NextRequest) {
           price = candleResult[0].close;
         }
       } else if (annotation.label_type === 'line' && annotation.geometry) {
-        const geometry = JSON.parse(annotation.geometry);
+        const geometry = annotation.geometry as any;
         price = geometry.startPrice || null;
       }